The presently disclosed embodiments are directed to an imaging member used in electrostatography. More particularly, the embodiments pertain to multi-layered electrophotographic imaging members prepared by using an identical polymer binder in the two contiguous imaging formation layers to eliminate the undesirable interfacial boundary effect and improves the imaging member photoelectrical performance. The embodiments also include a process for making and using the imaging member.
In electrophotographic reproducing apparatuses, including digital, image on image, and contact electrostatic printing apparatuses, a light image of an original to be copied is typically recorded in the form of an electrostatic latent image upon a photosensitive member and the latent image is subsequently rendered visible by the application of electroscopic thermoplastic resin particles and pigment particles, or toner. Multi-layered electrophotographic imaging members which are commonly employed in electrophotographic (xerographic) processing systems are well known in the art. They are typically prepared to have two distinctive configurations, namely: (1) the flexible electrophotographic imaging member belts (belt photoreceptors) and (2) the rigid electrophotographic imaging members in drum configuration.
Although the scope of the present embodiments covers the preparation of both flexible and rigid types of electrophotographic imaging members, the discussion hereinafter will focus on the flexible electrophotographic imaging members for purposes of simplicity.
In conventional prior art electrophotographic flexible imaging members, there may be included a photoconductive layer including a single layer or composite layers. One type of composite photoconductive layer used in xerography is illustrated in U.S. Pat. No. 4,265,990 which describes an imaging member having at least two electrically operative layers. One layer comprises a photoconductive layer or charge generating layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are supported on a conductive layer, the charge generating layer is sandwiched between a contiguous charge transport layer and the supporting conductive layer. Alternatively, the charge transport layer may be sandwiched between the supporting electrode and a charge generating layer.
In the case where the charge generating layer is sandwiched between the outermost exposed charge transport layer and the electrically conducting layer, the outer surface of the charge transport layer is charged negatively and the conductive layer is charged positively. The charge generating layer then should be capable of generating electron hole pair when exposed image wise and inject only the holes through the charge transport layer. In the alternate case when the charge transport layer is sandwiched between the charge generating layer and the conductive layer, the outer surface of the charge generating layer is charged positively while conductive layer is charged negatively and the holes are injected through from the charge generating layer to the charge transport layer. The charge transport layer should be able to transport the holes with as little trapping of charge as possible. In flexible imaging member belt such as photoreceptor, the charge conductive layer may be a thin coating of metal on a flexible substrate support layer.
Typical negatively charged imaging member belts, such as flexible photoreceptor belt designs, are made of multiple layers comprising a flexible supporting substrate, a conductive ground plane, a charge blocking layer, an optional adhesive layer, a charge generating layer, a charge transport layer. The charge transport layer is usually the last layer, or the outermost layer, to be coated and is applied by solution coating then followed by drying the wet applied coating at elevated temperatures of about 120° C., and finally cooling it down to ambient room temperature of about 25° C. When a production web stock of several thousand feet of coated multilayered imaging member material is obtained after finishing solution application of the charge transport layer coating and through drying/cooling process, upward curling of the multilayered photoreceptor is observed. This upward curling is a consequence of thermal contraction mismatch between the charge transport layer and the substrate support. Since the charge transport layer in a typical imaging member has a coefficient of thermal contraction approximately 3.7 times greater than that of the flexible substrate support, the charge transport layer does therefore have a larger dimensional shrinkage than that of the substrate support as the imaging member web stock cools down to ambient room temperature. Since the typical flexible electrophotographic imaging member, if unrestrained, exhibits undesirable upward imaging member curling, an anticurl back coating, applied to the backside, is required to balance the curl. Thus, the application of anticurl back coating is necessary to provide the appropriate imaging member belt with desirable flatness.
Flexible electrophotographic imaging members having these electrically operative layers, as disclosed above, provide electrostatic latent images when charged in the dark with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely divided electroscopic marking particles. The resulting toner image is usually transferred to a suitable receiving member such as paper or to an intermediate transfer member which thereafter transfers the image to a receiving member such as paper.
However, when a negatively charged imaging member (e.g., in belt configuration) is in dynamic cyclic motion under a normal machine operation condition in the field, the anticurl back coating of conventional imaging members (as the outermost exposed backing layer) is subject to high surface contact friction when it slides and flexes over the machine subsystems of the belt support module, such as rollers, stationary belt guiding components, and backer bars. The mechanical/frictional sliding interactions of ACBC against the belt support module components have been found to create numbers of issues; such as: (1) exacerbate ACBC wear/abrasion, causing loss of anti-curling control capability and resulting in imaging member belt curling-up problem because the thinning of the ACBC reduces its curl control effectiveness to result in premature curling up of the imaging member that impacts normal imaging belt machine functioning condition, such as non-uniform charging for proper latent image formation; (2) create debris/dirt of ACBC wear-off that scatters and deposits on critical machine components such as lenses; (3) wear/abrasion/scratch damage in the ACBC does also produce unbalanced forces between the charge transport layer and the ACBC to cause micro belt ripples formation during electrophotographic imaging process; (4) cause the development of tribo-electrical charge built-up in the ACBC that increases belt drive torque and, in some instances, it has been found to result in belt stalling; (5) in other cases, the tribo-electrical charge build up can be so high as to cause sparking; and lastly (6) under extensively cycled condition in precision electrostatographic imaging machines, an audible squeaky sound generation due to high contact friction interaction between the ACBC and the backer bars has also been a problem. Therefore, premature ACBC failure shortens the imaging member belt functional life and requires frequent costly belt replacement in the field. Moreover, inclusion of an ACBC to provide flatness also incurs additional material and labor cost.
In the recent development of curl free imaging members, a charge transport layer composition for making a flexible imaging member having substantial flatness without the need of an anticurl back coating and free of the fore-mentioned deficiencies) has been demonstrated through the external charge transport layer plasticization process. This process is accomplished by incorporation of a selected high boiler liquid plasticizer into the charge transport layer composition to reduce the internal stress/strain in the layer for effective effect curl control. The resulting imaging member is flat without an anticurl back coating and provides reduced production cost and extended service life without the problems suffered by the conventional imaging members.
The high boiler liquid plasticizer selected for the charge transport layer must meet the following: (i) the plasticizer is compatible during physical mixing with both the charge transport compound and the polymer binder to ensure the photo-electrical function integrity of the resulting imaging member and (ii) the plasticizer has a boiling point exceeding 250° C., so it is permanently present in the layer without (or negligible) loss due to evaporation during the functional life of the imaging member.
The term charge transport layer external plasticization process is defined as adding a plasticizer into the material matrix of the layer via a physical mixing without being chemically bound to either the charge transport compound or polymer binder, so that the plasticizer provides the effect for Tg reduction of the charge transport layer to suppress internal stress/strain build-up in the layer. Since the plasticizer, added is physical mixed with the charge transport layer components to effect Tg reduction, the external plasticization process, by definition, differs from that of internal plasticization process in which the added plasticizer is chemically bound to the polymer in the layer through a copolymerization reaction. See Ferdinand Rodrigues, Principles of Polymer Systems, Taylor & Francis Publisher (1996), pp. 58 to 59, and European Polymer Journal 44 (2008), pp. 366-375.
In the conventional prior art imaging member designs, the charge transport layer and the charge generation layer are respectively coated by using different polymer binders. However, the use of a different polymer binder to form the charge transport layer and charge generation layer creates a region of material discontinuity at the interface between these two contiguous layers. The existence of this discontinuity has two undesirable effects: first, it has inadequate adhesion bounding in this region so that, under normal dynamic imaging member belt cycling function in the machine, occasional premature delamination/separation failure has been found to cut short imaging member belt service life; second, the material discontinuity in this region also impedes charge transporting efficiency to adversely impact photoelectrical function. This is known as the interfacial boundary effect. Thus, it is desired to provide an imaging member that eliminates the region of discontinuity between the charge generation and charge transport layers.
Conventional photoreceptors are disclosed in the following patents, a number of which describe the presence of light scattering particles in the undercoat layers: Yu, U.S. Pat. No. 5,660,961; Yu, U.S. Pat. No. 5,215,839; and Katayama et al., U.S. Pat. No. 5,958,638. The term “photoreceptor” or “photoconductor” is generally used interchangeably with the terms “imaging member.” The term “electrostatographic” includes “electrophotographic” and “xerographic.” The terms “charge transport molecule” are generally used interchangeably with the terms “hole transport molecule.”
Yu, U.S. Pat. No. 6,660,441, issued on Dec. 9, 2003, discloses an electrophotographic imaging member having a substrate support material which eliminates the need of an anticurl backing layer, a substrate support layer and a charge transport layer having a thermal contraction coefficient difference in the range of from about −2×10−5/° C. to about +2×10−5/° C., a substrate support material having a glass transition temperature (Tg) of at least 100° C., wherein the substrate support material is not susceptible to the attack from the charge transport layer coating solution solvent and wherein the substrate support material is represented by two specifically selected polyimides.
In U.S. Pat. No. 7,413,835 issued on Aug. 19, 2008, it discloses an electrophotographic imaging member having a thermoplastic charge transport layer, a polycarbonate polymer binder, a particulate dispersion, and a high boiler compatible liquid. The disclosed charge transport layer exhibits enhanced wear resistance, excellent photoelectrical properties, and good print quality.
In U.S. application Ser. No. 10/982,719, filed on Nov. 5, 20024 entitled “Imaging Member,” by Yu et al., discloses an imaging member formulated with a liquid carbonate. The imaging electrostatographic member exhibits improved service life.